In a conventional delayed coking process, petroleum residues are fed at elevated temperatures to a large steel vessel called a coke drum, where the residues are thermally cracked and formed into coke. When the coke drum has filled with coke, the coke drum is injected with steam and cooled with water. To remove the coke from the coke drum, top and bottom covers of the drum are removed in a process called unheading.
Since the coke drum must contain a severe atmosphere of elevated temperatures and high pressure, the bottom cover is typically secured to the coke drum by dozens of bolts that often must be loosened manually.
Reducing the labor involved in unheading is typically accomplished using remotely-operated actuator systems. These systems are less labor-intensive since removing the bolts and cover can be automated.
However, actuator systems also have some drawbacks. For example, if the components of the system are welded to the coke drum, the welds may deteriorate over time because of the high operating temperatures of the drum. Moreover, the weight of the contents of the drum, which the actuator system must handle, also adds a load to the actuator system, which it may not have been designed to withstand. A further drawback of existing systems is their lack of stability. When removing the massive cover, it is important that the head remain securely held throughout the unheading process. So far, no proposal has mitigated all these concerns.
One proposal is shown in U.S. Pat. No. 5,098,524 to Antalfy et al. Antalfy et al. shows a coke drum unheading device in which the bottom cover remains attached to the coke drum throughout unheading. A bottom cover 27 is connected to the drum 1 by way of a pivot plate 35. A pair of hydraulic actuators 22 unhinges the bottom cover 27 from above. The drawback of this arrangement is that it attaches the pivot plate to the coke drum 1 itself. As a result, the weld holding the pivot plate 35 to the drum is exposed to the heat of the coke drum during coking, which reduces the life of the weld and therefore increases maintenance cost.
U.S. Pat. No. 5,336,375 to Wallskog et al. attempts to remedy the weld problem by an embodiment shown in FIGS. 5 and 6. As shown, the cylinders 162, 163 are attached to a removable support structure 180. Thus, the cylinders 162, 163 need not be welded to the coke drum. Nevertheless, the arrangement is less rigid than one in which the cylinders are attached to the coke drum itself, so cables 174 are needed to resist bending moments on the support structure 180. The drawback is that the support structure must withstand the additional weight of the coke drum and its contents as well as the large cables needed to add strength to the support structure. This may shorten the life of the coke drum, or may be impossible to install in existing coke drums not designed for such stresses.
Another proposal is shown in U.S. Pat. No. 5,500,094 to Fruchtbaum et al. Fruchtbaum et al. shows a coke drum unheading device in which a bottom cover 12 has a protrusion 54. This protrusion 54 mates with notch 52 in a bearing plate 40. A hydraulic actuator 34 is carried on a car 22 and brings the bearing plate 40 up against the bottom cover 12. When the bolts holding the bottom cover 12 to the coke drum flange 14 are loosened, gravity presses the cover 12 against bearing plate 40. Although Fruchtbaum et al. does not attach any of the components to the drum, it does use a precarious system to remove the cover, reducing stability.
Each past proposal has its advantages, but the disadvantages of each indicate a need in the art for an actuator system that is not welded to the coke drum, is not supported by the drum, does not stress the coke drum with additional weight and will operate in low-clearance environments.